This invention relates to cathode ray tube fabrication and more particularly to a process for removing undesired gases from within a cathode ray tube envelope without deleterious effects upon the cathode ray tube assembly.
Generally, cathode ray tubes include an envelope having a funnel-portion and a neck portion with a viewing screen affixed to one end and the neck portion affixed to the opposite end of the funnel portion. An electron gun assembly is sealed into the neck portion and usually includes a plurality of electron guns aligned in either a delta or an in-line configuration. Also, each electron gun has a potentially electron emissive coating thereon including a mixture of carbonates held together by a binder and affixed thereto prior to processing of the cathode ray tube.
In processing the cathode ray tube, the envelope containing the viewing screen is deposited onto a sealing machine. The electron gun assembly is also deposited onto the sealing machine. The sealing machine positionally locates the electron gun assembly within the neck portion of the cathode ray tube. Thereafter, heat is applied to the junction of the electron gun assembly and the envelope of the cathode ray tube in an amount sufficient to effect a glass seal therebetween.
During the above-mentioned sealing process, it has been found that temperatures within the envelope reach the range of about 300.degree.-450.degree. C. Since the most common form of binder material utilized with carbonate mixtures to provide coatings for electron gun cathodes is in the form of a nitrocellulose and nitrocellulose tends to decompose at the above-mentioned range of temperatures, it has been found that the coating on the electron gun cathodes after the sealing process has been completed is in the form of a relatively weakly bonded carbonate mixture.
Also, it has been found that the weakly bonded carbonate mixture is readily susceptible to damage during the exhaust cycle of the cathode ray tube fabricating process. Specifically, it has been found that the envelope of the cathode ray tube includes water vapor and upon evacuation of the envelope the water vapor is condensed to provide droplets of water. In turn, the water droplets are rapidly drawn toward the weakly bonded carbonate mixture on the cathode electrode during the evacuation process and tend to seriously damage the carbonate mixture which is obviously deleterious to the cathode ray tube structure.
In one known attempt to alleviate the above-mentioned unacceptable conditions, a second binder having a higher decomposition temperature was mixed with the first binder and employed to affix a suspension of carbonates to the cathode structure. The second binder was a synthetic resin such as a mixture of condensation products of phenol and farfural and is resistant to temperatures in the 300.degree.-450.degree. C range and decomposes at temperatures in the range of about 1200.degree. C.
Another known attempt to inhibit a deleterious effect on the cathode coating during the exhaust cycle is set forth in U.S. Pat. No. 3,978,563 issued on Sept. 7, 1976 to Schol et al. Therein, a dual binder system is employed with a first binder of the ordinary nitrocellulose type and a second binder selected from the group consisting of polyimides, highly molecular resins and nylon types.
Although each of the above-mentioned double-binder techniques appears to offer some relief to the untenable damage to the cathode coating during the exhaust cycle, it was found that each leaves something to be desired. For example, dual binder systems are expensive to provide, cumbersome to utilize, the tend to cause a multitude of viscosity and deterioriation problems.